A photolithography system has as its basic components an illuminator with a light source, a patterned reticle, a projection lens and a photosensitive (e.g., photoresist-coated) wafer. The illuminator illuminates the reticle with light from the light source. Light transmitted by or reflected from the reticle is then imaged by the projection lens onto the photosensitive wafer. The photosensitive wafer is then processed to form a pattern on the wafer. The photolithographic exposure process and post-exposure process is repeated with a number of different reticles to form on the wafer a semiconductor structure that defines an integrated circuit.
The photolithography system needs to provide a precise amount of exposure energy (i.e., an “exposure dose”) to the photosensitive wafer over a well-defined region (i.e., “exposure field”). Conventional photolithography systems typically include a shutter formed by mechanical blades placed in the optical path within the illuminator to define the exposure dose. A mechanical aperture is also used to define the dimensions of the exposure field.
It is important that the photolithography system maintains the correct exposure dose so that the photolithography process has the largest process window with respect to resolution and depth of focus. The ability to maintain the proper exposure dose depends on the shutter timing and in particular the shutter switching, i.e., the off/on/off transition timing. For low exposure dose requirements, the amount of time the shutter is open becomes comparable to or even smaller than the amount of time needed to switch the shutter blades. There is no effective way to control that shutter-blade switching time apart from dramatically increasing the speed and acceleration of the shutter blades. However, this approach is limited by mechanical considerations. The increased acceleration and deceleration required cause vibrations in the optical system that adversely affect the performance of the photolithography system. Thus, using a conventional mechanical shutter to make small changes in the exposure dose without adversely affecting the operation of the photolithography system is a challenge.
Further, a photolithography system may need to accommodate reticles of different sizes. The patterned region of the reticle is called the reticle field. The reticle field has dimensions that correspond to the dimensions of the exposure field at the wafer. The exposure of the reticle field should not result in an exposure larger than the intended exposure field. The photolithography system thus usually includes the aforementioned mechanical aperture to define the exposure field dimensions. The mechanical aperture must be adjusted to physically match the size requirements of the exposure field. As the mechanical aperture includes mechanically driven blades, it can be a source of system failure and can adversely impact the mean time between failures (MTBF) of the photolithography system.
Some photolithographic exposures require that an annular portion of the photoresist (e.g., 2 to 4 mm) along the edge of the wafer remain unexposed. This may be the case for wafers that need to undergo an electroplating step after the photolithography exposure step. Normally, the photoresist on some of the exposure fields on the wafer ends up being exposed all the way out to the edge of the wafer. Presently, a mechanical ring system is used to prevent exposure of the photoresist near the wafer edge. However, this mechanical solution is not robust and is subject to failure, which can adversely impact the MTBF of the photolithography system.